Various bulk wave resonators that utilize bulk waves propagating through a piezoelectric thin film have been proposed. For example, Patent Document 1 discloses an acoustic resonator filter that includes a first electrode on a first surface of a piezoelectric sheet and a second electrode on a second surface of the piezoelectric sheet. This acoustic resonator filter utilizes bulk waves that propagate through a piezoelectric sheet. In Patent Document 1, an overlap between the first electrode and the second electrode has an irregular polygonal shape so as to reduce transverse-mode spurious modes.
Patent Document 2 discloses a piezoelectric thin-film resonator that includes a mass addition film on an electrode in order to reduce spurious modes.
Patent Document 1:Japanese Unexamined Patent Application Publication No. 2000-332568
Patent Document 2:WO 2007/119556